Damped control of a micromechanical device

ABSTRACT

Device and method for a damping function to reduce undesirable mechanical transient responses to control signals. In one aspect of the present invention, the damping function may be used to reduce overshoot and oscillation when a digital micromirror is driven from a landing plate to the flat or neutral position. In another aspect of the present invention, the damping function may be used to reduce transient resonance of a digital micromirror when the micromirror is driven to a landing plate.

[0001] This application claims the priority benefit of U.S. ProvisionalApplication Ser. No. 60/472,079, entitled “Damped Reset for a DigitalMicromirror Device,” filed on May 20, 2003, which application is herebyincorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application relates to the following co-filed and commonlyassigned patent applications: Ser. No. ______ [TI-34206], entitled“Damped Control of a Micromechanical Device;” and Ser. No. ______[TI-34448], entitled “Damped Control of a Micromechanical Device,” whichapplications are hereby incorporated herein by reference.

TECHNICAL FIELD

[0003] The present invention relates generally to micro-mechanicaldevices, and more particularly to a device and method for damping themovable elements of such devices.

BACKGROUND

[0004] Generally, video display systems based on spatial lightmodulators (SLMs) are increasingly being used as an alternative todisplay systems using cathode ray tubes (CRTs). SLM systems generallyprovide high resolution displays without the bulk and power consumptionof CRT systems.

[0005] One type of SLM is a digital micromirror device (DMD). Digitalmicromirror devices have also been called deformable micromirrordevices, although that term generally is now used to describe devicesthat operate in an analog mode. DMDs may be used for either direct-viewor rear or front projection display applications. A DMD has an array ofmicro-mechanical display elements, each having a tiny mirror that isindividually addressable by an electronic signal. Depending on the stateof its addressing signal, each mirror tilts so that it either does ordoes not reflect light to the image plane. The mirrors may be generallyreferred to as “display elements,” which correspond to the pixels of theimage that they generate. Generally, displaying pixel data isaccomplished by loading memory cells connected to the display elements.After a display element's memory cell is loaded, the display element isreset so that it tilts in the on or off position represented by the newdata in the memory cell. The display elements are able to maintain theon or off state for controlled display times.

[0006] In the prior art, a reset sequence generally is used to drive aDMD pixel. The reset sequence generally comprises 5 steps including (1)memory write, (2) reset, (3) release and differentiation, (4) landingand latch, and (5) stabilization. This cycle, including mechanicalswitching time, generally takes about 15 μsec to complete. While a biasvoltage is applied, the micromirror is electro-mechanically latched,allowing update of memory data for the next reset sequence.

[0007] One potential disadvantage with the mechanical nature of a DMDmirror is that a transient response typically is seen just after themirror tip (spring tip) lands on a landing plate. Then the responsegradually decays in accordance with a receiving resistance from ambientgas at about one atmosphere pressure. The resonant vibration seen in atransient response generally originates from the physical structure ofthe micromirror. A transient frequency of the mirror may be about450-550 kHz, although the value may vary depending on the specificapplication. Data loading to an SRAM under the micromirror generally isperformed after the response declines to a level that is low enough toguarantee the prevention of a micromirror malfunction. Therefore astabilization period is required to remove mechanical instability in theswitching operation of a micromirror. This time may take up two-thirdsof the mechanical switching time.

[0008] In general, the total response time is composed of the mechanicalswitching time and the data loading time for a mirror section, whichtime determines the minimum cycle time. This directly relates to an LSBtime for binary pulse width modulation techniques in controlling graylevels, which generally determines the color bit depth on a DMD system.

[0009] Generally, bit depth requirements are increasing the need forshorter LSB times on SLM projectors. In the prior art, a fast cleartechnique has been used to achieve high bit depth. Generally, fast clearis a function whereby all mirrors are sent to a flat state by a singlevoltage change. Fast clear, however, may be reliable to only as low asabout 14 μsec t-wait time (approximately 15 μsec cycle time). Moreover,even at that level, there may be a significant yield loss.

[0010] Alternatively, a reset-release function has been used in theprior art to achieve high bit depth. The reset-release technique canprovide a shorter cycle time (e.g., 8 μsec) than the fast cleartechnique. A reset-release pulse is used to cause a display element toassume an unaddressed flat state, in which the display element “floats.”During this float time, the next address state is loaded. Then a biasvalue is reapplied and the display element assumes the new addressstate. The use of reset-release display times is described in U.S. Pat.No. 5,764,208 entitled “Reset Scheme for Spatial Light Modulators,” andU.S. Pat. No. 6,008,785, entitled “Generating Load/Reset Sequences forSpatial Light Modulators,” each of which patents is assigned to TexasInstruments Incorporated and incorporated herein by reference.

[0011] One potential disadvantage with the prior art reset-release pulsesequence is that it may introduce a significant loss in contrast (e.g.,about 30%). In addition, it may introduce a potential residual imageartifact with hinge memory over the device lifetime. These disadvantagesgenerally are due to a significant overshoot of the mirror into thepupil of the optical system after launch.

SUMMARY OF THE INVENTION

[0012] These and other problems are generally solved or circumvented,and technical advantages are generally achieved, by preferredembodiments of the present invention which utilize a damping function toreduce undesirable mechanical transient responses to control signals. Inone aspect of the present invention, the damping function may be used toreduce overshoot and oscillation when a DMD mirror is driven from alanding plate to the flat or neutral position. In another aspect of thepresent invention, the damping function may be used to reduce transientresonance of a DMD mirror when the mirror is driven to a landing plate.

[0013] In accordance with a preferred embodiment of the presentinvention, a method for operating a digital micromirror device having atleast one micromirror comprises applying a reset voltage pulse to themicromirror. The reset voltage causes the micromirror to launch from alanding plate. Then an offset voltage is applied to the micromirrorimmediately after the reset voltage pulse. The offset voltage is appliedfor a damping delay period. The micromirror launches and is moving awayfrom the landing plate before an expiration of the damping delay period.A damping pulse is applied to the micromirror immediately after theoffset voltage, and then the offset voltage is reapplied to themicromirror. The damping pulse generally reduces oscillation of themicromirror about the neutral position. In one preferred embodiment, thedamping pulse voltage is equal to the reset voltage, while in anotherpreferred embodiment the damping pulse voltage is equal to a biasvoltage for the micromirror.

[0014] In accordance with another preferred embodiment of the presentinvention, a system for operating a digital micromirror device having atleast one micromirror comprises applying a reset voltage pulse to themicromirror, then applying an offset voltage to the micromirrorimmediately after the reset voltage pulse. A bias voltage is applied tothe micromirror immediately after the offset voltage. The bias voltageis applied for a damping delay period. A triangular damping pulse isapplied to the micromirror after the damping delay period, and then thebias voltage is reapplied to the micromirror. The triangular dampingpulse generally reduces a transient resonant vibration of themicromirror on a landing plate.

[0015] An advantage of a preferred embodiment of the present inventionis that it provides more stable and reliable mechanical switchingoperation, and enables shorter mechanical switching time that allowsless minimum expose time and increased color bit depth. This in turn mayprovide a reduction in video noise and contouring.

[0016] A further advantage of a preferred embodiment of the presentinvention is that it provides increased yield. In particular, while theLSB may be a damped reset-release bit, the next bit, which may be fastclear, may be significantly longer. Thus, fast clear wait time may beincreased, which relaxes the criteria for yield.

[0017] Another advantage of a preferred embodiment of the presentinvention is that there generally is no need to change the current DMDpixel design, structure, or process (except for modifying controlfunctions to implement the damping function).

[0018] Yet another advantage of a preferred embodiment of the presentinvention, when used to dampen mirror resonance on landing on a landingplate, is that there may be less mechanical friction between the mirrorlanding tip and the mirror landing plate, resulting in less mechanicalwear on the passivation layer.

[0019] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

[0021]FIG. 1 is a block diagram of a spatial light modulator displaysystem;

[0022]FIG. 2 is an exploded perspective view of a DMD mirror element;

[0023]FIG. 3 is a perspective view of another type of DMD mirrorelement;

[0024]FIGS. 4a-b are graphs of a reset-release sequence and mirrorposition response;

[0025]FIG. 5 is a graph of a damped reset-release sequence;

[0026]FIGS. 6a-6 f are a graphical representation of a reset-releasesequence applied to a micromirror device;

[0027]FIG. 7 is a graph comparing mirror position response of anundamped reset-release sequence to that of a damped reset-releasesequence;

[0028]FIGS. 8a-b are graphs of a damped reset-release sequence;

[0029]FIGS. 9a-b are graphs of mirror position response for a standardreset-release sequence and a damped reset-release sequence,respectively;

[0030]FIGS. 10a-b are graphs of black video content for a standardreset-release sequence and a damped reset-release sequence,respectively;

[0031]FIG. 11 is a graph of a damped reset-release sequence;

[0032]FIG. 12a is a graph of the time and frequency domain functions fora triangular pulse;

[0033]FIG. 12b is a graph of the time and frequency domain functions fora rectangular pulse;

[0034]FIGS. 13a-b are damped reset sequences with one and two triangularpulses, respectively;

[0035]FIG. 14 is a graphical side elevation view of a DMD mirrorelement; and

[0036]FIG. 15 is a graph of a damped reset sequence and its associatedmirror angle response.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0037] The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

[0038] The present invention will be described with respect to preferredembodiments in a specific context, namely a DMD with a damped reset andrelease. The invention may also be applied, however, to other types ofspatial light modulators, to other micro-electromechanical system(“MEMS”) devices having movable elements that move in response toelectrostatic attraction, or to other MEMS devices utilizing transientcontrol of movable elements.

[0039] One application of DMDs is for forming images where the DMD hasan array of deflectable mirrors that selectively reflect light to animage plane. The images formed by the DMD can be used in display systemsor for non-impact printing applications. Other applications of DMDs arepossible that do not involve image formation, such as optical steering,optical switching, and accelerometers. In some of these applications,the mirror element need not be reflective. Also, in some applications,the DMD is operated in an analog rather than a digital mode. In general,the term “DMD” is used herein to include any type of micro-mechanicaldevice having at least one hinge-mounted deflectable element that isspaced by an air gap from a substrate, relative to which it moves.

[0040] With reference now to FIG. 1, there is shown an example of atypical projection display system 10. System 10 uses SLM 15 to generatereal-time images from an input signal, such as a broadcast televisionsignal. In the example of this description, the input signal is analog,but in other embodiments the input signal could be digital, eliminatingthe need for A/D converter 12 a. Generally, only those componentssignificant to main-screen pixel data processing are shown. Othercomponents, such as might be used for processing synchronization andaudio signals or secondary screen features, such as closed captioning,are not shown.

[0041] Each display element of the SLM 15 has a memory cell, which isloaded with one bit of data at a time. The one bit of data in memory forall display elements comprises a bit-plane. The instance of displaying agiven bit-plane is referred to herein as a “segment,” and a bit-planemay be displayed in one continuous segment or in multiple segmentsdistributed throughout a frame.

[0042] After its memory cell is loaded, the display element is reset tothe state represented by the data in the memory cell. This loading andresetting process occurs in a particular sequence of loads and resetsgenerated by sequence controller 18. Sequence controller 18 deliverscontrol signals following this sequence to the frame memory 14 forloading and to SLM 15 for resetting.

[0043] In one implementation, system 10 may have a divided resetconfiguration. The sequence generation process classifies segmentsaccording to the length of their initial display times. It thenallocates actual display times so that segments having shorter displaytimes can be loaded during a prior segment. The shortest of these shortdisplay times are treated as “reset-release” display times, which callfor reset-releases in the sequence. It also prevents reset conflictsbetween reset sequences of any two or more groups. In anotherimplementation, system 10 may have a global reset system. In the case ofa global reset system, using concepts similar to those discussed above,the process may classify segments and provide for short andreset-release display times.

[0044] Signal interface unit 11 receives an analog video signal andseparates video, synchronization, and audio signals. It delivers thevideo signal to A/D converter 12 a and Y/C separator 12 b, which convertthe data into pixel-data samples and which separate the luminance (“Y”)data from the chrominance (“C”) data, respectively. In FIG. 1, thesignal is converted to digital data before Y/C separation, but in otherembodiments, Y/C separation could be performed before A/D conversion.

[0045] Processor system 13 prepares the data for display by performingvarious pixel data processing tasks. Processor system 13 may includewhatever processing memory is useful for such tasks, such as field andline buffers. The tasks performed by processor system 13 may includelinearization to compensate for gamma correction, colorspace conversion,and interlace to progressive scan conversion. The order in which thesetasks are performed may vary.

[0046] Display memory 14 receives processed pixel data from processorsystem 13. It formats the data, on input or on output, into “bit-plane”format, and delivers the bit-planes to SLM 15. The bit-plane formatpermits each display element of SLM 15 to be turned on or off inresponse to the value of one bit of data. Display memory 14 may providebit-plane data to be displayed on the rows of the SLM that areassociated with a designated group.

[0047] In one implementation of display system 10, display memory 14 isa “double buffer” memory, which means that it has a capacity for atleast two display frames. The buffer for one display frame can be readout to SLM 15 while the buffer for another display frame is beingwritten. The two buffers are controlled in this manner so that data iscontinuously available to SLM 15. SLM 15 uses the data from displaymemory 14 to address each display element of its display element array.

[0048] Display optics unit 16 has optical components for receiving theimage from SLM 15 and for illuminating an image plane such as a displayscreen. For color displays, the display optics unit 16 includes a colorwheel, to which a sequence of bit-planes for each color is synchronized.In an alternative embodiment, the bit-planes for different colors couldbe concurrently displayed on multiple SLMs and combined by the displayoptics unit.

[0049] Master timing unit 17 may provide various system controlfunctions to sequence controller 18 and may provide system controlfunctions to other components in system 10. As discussed previously,sequence controller 18 provides reset control signals to SLM 15 and loadcontrol signals to display memory 14.

[0050] With reference to the display elements in SLM 15, FIG. 2 is anexploded perspective view of a single mirror element 20 of a DMD. InFIG. 2, mirror 21 is shown undeflected, but as indicated by the arrows,its torsion hinges 22 permit it to be deflected in either of twodirections.

[0051] Mirror element 20 of FIG. 2 generally is known as a “hiddenhinge” mirror element. Other types of mirror elements 20 may befabricated, including a “torsion beam” type, described below inconnection with FIG. 3, where the mirror is mounted directly to thehinges instead of over a yoke to which the hinges are attached. VariousDMD types are described in U.S. Pat. No. 4,662,746, entitled “SpatialLight Modulator and Method”; U.S. Pat. No. 4,956,610, entitled “SpatialLight Modulator”; U.S. Pat. No. 5,061,049 entitled “Spatial LightModulator and Method”; U.S. Pat. No. 5,083,857 entitled “Multi-LevelDeformable Mirror Device”; and U.S. Pat. No. 5,583,688, entitled“Improved Multi-Level Micro-Mirror Device.” Each of these patents isassigned to Texas Instruments Incorporated and each is incorporatedherein by reference.

[0052] As with other hidden hinge DMD designs, hinges 22 of mirrorelement 20 are supported by hinge support posts 23, which are formed ona substrate. Address electrodes 24 are supported by address electrodesupport posts 25, which are on the same level as hinges 22 and hingesupport posts 23.

[0053] Mirror support post 26 is fabricated on yoke 27. Yoke 27 isattached to one end of each of the two hinges 22. The other end of eachhinge 22 is attached to a hinge support post 23. The hinge support posts23 and the electrode support posts 25 support hinges 22, addresselectrodes 24, and yoke 27 over a substrate having a control bus 28 a.When mirror 21 is tilted, the tip of mirror 21 contacts a landing site29. Address electrodes 24 have appropriate electrical connections tomemory cells (not shown), which are typically fabricated withinsubstrate 28 using CMOS fabrication techniques.

[0054]FIG. 3 illustrates mirror element 30 of a “torsion beam” type DMD.In this type of DMD, hinges 32 are not hidden, but rather extend fromopposing sides of mirror 31. Hinges 32 are attached to hinge supportposts 33. Address electrodes 36 provide attractive forces for tiltingmirror 31, which touches a landing pad 37. Mirror element 30 isfabricated over a substrate 38 of memory cells and control circuitry.

[0055] In operation for image display applications, and using an arrayof mirror elements 30 for example, a light source illuminates thesurface of the DMD. A lens system may be used to shape the light toapproximately the size of the array of mirror elements 30 and to directthis light toward them. Voltages based on data in the memory cells ofsubstrate 38 are applied to the address electrodes 36. Electrostaticforces between the mirrors 31 and their address electrodes 36 areproduced by selective application of voltages to the address electrodes36. The electrostatic force causes each mirror 31 to tilt either about+10 degrees (on) or about −10 degrees (off), thereby modulating thelight incident on the surface of the DMD. Light reflected from the “on”mirrors 31 is directed to an image plane, via display optics. Light fromthe “off” mirrors 31 is reflected away from the image plane. Theresulting pattern forms an image. The proportion of time during eachimage frame that a mirror 31 is “on” determines shades of gray. Aspreviously discussed, color can be added by means of a color wheel or bya three-DMD setup.

[0056] In effect, the mirror 31 and its address electrodes 36 formcapacitors. When appropriate voltages are applied to mirror 31 and itsaddress electrodes 36, a resulting electrostatic force (attracting orrepelling) causes the mirror 31 to tilt toward the attracting addresselectrode 36 or away from the repelling address electrode 36. The mirror31 tilts until its edge contacts landing pad 37.

[0057] Once the electrostatic force between the address electrodes 36and the mirror 31 is removed, the energy stored in the hinges 32provides a restoring force to return the mirror 31 to an undeflected orequilibrium position. Appropriate voltages may be applied to the mirror31 or address electrodes 36 to aid in returning the mirror 31 to itsequilibrium position. Various types of voltages and waveforms that maybe applied to the mirror are described in U.S. Pat. No. 6,008,785,entitled “Generating Load/Reset Sequences for Spatial Light Modulator”;U.S. Pat. No. 5,912,758, entitled “Bipolar Reset for Spatial LightModulator”; U.S. Pat. No. 5,771,116, entitled “Multiple Bias Level ResetWaveform for Enhanced DMD Control”; U.S. Pat. No. 5,768,007, entitled“Phase Matched Reset for Digital Micro-Mirror Device”; U.S. Pat. No.5,764,208, entitled “Reset Scheme for Spatial Light Modulators”; andU.S. Pat. No. 5,706,123, entitled “Switched Control Signals for DigitalMicro-Mirror Device with Split Reset,” each of which patents is assignedto Texas Instruments Incorporated and each of which is incorporatedherein by reference.

[0058] An example of a prior art reset-release waveform is shown in FIG.4a. Prior to executing a reset-release sequence, a DMD element has abias voltage 40 applied to it, during which time the mirror is tiltedat, e.g., about 10 degrees. A reset pulse 42 is applied to cause themirror to return to a flat position, or about 0 degrees. Following thereset pulse is offset voltage 44, which allows a new address state to beloaded while the mirror is in a floating position. After this, a biasvoltage 46 is again applied so that the DMD element may assume the newaddress state.

[0059] An expanded view of a portion 48 of the reset-release sequence,and its effect on mirror angle position 49, is shown in FIG. 4b.Initially, bias voltage 40 is applied, and the mirror is tilted at anangle of about 11-12 degrees. Reset pulse 42 causes the mirror to breakfree from surface stiction force and launch toward a neutral position.The reset pulse is followed by offset voltage 44 to complete thereset-release sequence. As can be seen in FIG. 4b, however, the mirrorovershoots 0 degrees and experiences significant oscillation beforesettling to the 0 degree position. The overshoot and oscillationgenerally is undesirable and may cause visual artifacts. In particular,it can create black level, contrast, light leakage, and residual imageproblems. In addition, the extended oscillation may necessitate a longsettling time before the bias voltage can be reapplied.

[0060] In accordance with a preferred embodiment of the presentinvention as shown in FIG. 5, these problems may be mitigated oreliminated by modifying the reset-release sequence with the addition ofa damping or braking pulse to create a damped reset-release sequence. Asbefore, bias voltage 50 is applied to a mirror element, followed by areset-release sequence initiated by reset pulse 52. Instead oftransitioning to a long offset voltage time period, a short dampingdelay 54 is introduced during which time an offset voltage is applied.The damping delay 54 allows sufficient time for the mirror to launch andachieve a small angular excursion from the original landing point. Whenthe damping delay 54 time period expires, a damping pulse 56 is appliedto the mirror element. The damping pulse 56 generates a retardationtorque on the departing mirror, thus reducing some of the potentialenergy stored within the mirror spring tip and hinge, plus energyinstilled by the reset pulse. This has the effect of damping down themagnitude of the oscillation around the neutral position, as well asreducing the mirror settling period. Depending on specific values chosenfor various parameters, the final motion of the mirror may beover-damped, critically damped, or still under-damped, but at a reducedlevel. After the damping pulse, an offset voltage 58 is again applied tothe mirror element, followed by the bias voltage 60 as before.

[0061]FIGS. 6a-f illustrate mirror position versus voltage forcesapplied to the mirror during a damped reset-release sequence. In FIGS.6a-f, the relative length of the arrows generally shows a qualitativerepresentation of the forces on the mirror. In FIG. 6a, mirror 60 istilted and on landing pad 62, with bias voltage 64 being applied to holdmirror 60 down on landing pad 62. In FIG. 6b, a reset voltage 65 isapplied to mirror 60, which will cause mirror 60 to break free andlaunch from landing pad 62. In FIG. 6c, an offset voltage 66 is appliedto mirror 60 for the damping delay time period, during which time mirror60 has started moving away from landing pad 62. After the damping delaytime period expires, a damped reset pulse 67 is applied to the mirror asshown in FIG. 6d. The damped reset pulse generally is applied aftermirror 60 has begun moving away from landing pad 62, but before mirror60 has traveled to the neutral position. Once the damped reset pulsetime period has finished, an offset voltage 68 is again applied to themirror as shown in FIG. 6e. The mirror 60 then settles to the neutralposition as shown in FIG. 6f.

[0062] Preferably, the widths of the damping delay and damping resetpulse are set to remove the most amount of energy from the mirror, whileensuring that there is enough margin against the mirror being calledback to the original landing surface. The specific values may bedetermined experimentally based on the steady state operation of amirror device. The parameters may be set such that the mirror travel tothe neutral position is over-damped, critically damped, or may beunder-damped but with reduced magnitude swings around the neutralposition.

[0063]FIG. 7 is a graph comparing an undamped reset-release sequencewith a damped reset-release sequence, and their respective effects onmirror displacement. Undamped reset-release sequence 70 and dampedreset-release sequence 72 both return to an offset voltage afterinjecting a reset pulse. Undamped reset-release sequence 70 remains atthe offset voltage for the remainder of the sequence. Dampedreset-release sequence 72, however, injects another pulse after adamping delay time period. Trace 74 represents the mirror displacementgenerated by the undamped reset-release sequence 70. As can be seen inFIG. 7, there is a large displacement of the mirror and the mirroroscillates with relatively high magnitude for an extended period oftime. In contrast, trace 76, which represents the mirror displacementgenerated by the damped reset-release sequence 72, has much smalleroscillations and dampens close to the final value much more quickly.

[0064] Variations of the damped reset-release waveform may be used inaccordance with the present invention. One example of an alternativeembodiment damped reset-release sequence is shown in FIGS. 8a-b, whereinFIG. 8b is an expanded view of portion 80 of the waveform shown in FIG.8a. In this embodiment, after a reset pulse 82 and a damping delay 83, adamping pulse 84 in the opposite direction from the reset pulse 82 isapplied to the mirror element. In this case, damping pulse 84 has thesame voltage as the bias voltage instead of the reset pulse. Also inthis case, the damping pulse has a polarity opposite that of the resetpulse. Damping pulse 84 provides an electrostatic force that opposes themotion of the pixel as it travels from the releasing side towards theflat state. Compared to the embodiment shown in FIG. 5, it also may havedecreased attraction toward the non-releasing side, thus providing morenet moment to damp the pixel oscillation.

[0065] Referring now to FIGS. 9a and 9 b, there is shown a comparison ofmirror angle response for a standard reset-release and a dampedreset-release. FIG. 9a illustrates standard reset-release waveform 90,and a model mirror angle response 92, which response has significantoscillation about the 0 degrees position before settling down to thatposition. In contrast, FIG. 9b illustrates damped reset-release waveform94, and a damped mirror angle response 96. As readily seen in acomparison of FIGS. 9a and 9 b, the addition of the damping pulse 98 tothe damped reset-release has the effect of significantly damping theoscillations of the mirror angle about the 0 degrees position.

[0066] In this embodiment, voltage parameter values have been selectedsuch that the reset voltage is −26 volts, the bias voltage is 24 volts,and the offset voltage is 7 volts. In addition, timing parameter valueshave been selected such that the reset pulse width is 0.6 microseconds,the damping delay time is 1.6 microseconds, and the damping pulse widthis 3.9 microseconds. Because the bias voltage is used for the dampingpulse, the damping pulse has a voltage of 24 volts, compared with thereset voltage value of −26 volts used in the embodiments of FIGS. 5 and7. Of course, as with other embodiments, the specific values for thevarious parameters associated with the damped reset-release waveform,such as the voltage levels for the bias voltage, offset voltage, initialreset voltage, damping pulse voltage and the time periods for therelease pulse time 86, damping delay time 87, and damping pulse time 88(as shown in FIG. 8b), may be varied depending on the particularapplication.

[0067] Another method of measuring the effect of mirror overshoot andoscillation is to measure the black level increase caused by thestandard reset-release and the damped reset-release. FIG. 10a is a graphof the standard reset-release and FIG. 10b is a graph of the dampedreset-release, with both graphs also illustrating the associated blackvideo level. As illustrated by FIG. 10a, standard reset-release function100 causes a substantial increase 104 in black video level 102, whichincrease is related to the amplitude of the first overshoot of themirror position past the off-state. Subsequent decreasing spikes in theblack level are caused by oscillation of the mirror position around theoff-state. In comparison, the damped reset-release function 106 of FIG.10b causes very little change in black video level 108.

[0068] In accordance with another preferred embodiment of the presentinvention, an alternative reset-release function 110 is shown in FIG.11. A reset pulse 114 follows bias voltage 112 as before. With thiswaveform, however, instead of using a damping delay at the offsetvoltage after the reset pulse, a damping pulse 116 is initiatedimmediately after the reset pulse 114. After damping pulse 116, thevoltage level returns to offset voltage 118 for the remainder of thereset-release function 110. Although this waveform function may beuseful in some applications, one potential problem with this embodimentin other applications is that this waveform may create too strong of aretarding moment and may not allow the mirror to successfully releasefrom the landing zone.

[0069] In accordance with another preferred embodiment of the presentinvention, a damping pulse sequence may be used to suppress the resonantvibration of a mirror on a landing plate. As previously discussed, theremay be a transient response introduced when a mirror is driven to alanding plate, either from that same landing plate (stay trajectory) orfrom the other landing plate (crossover trajectory). The transientresponse generally depends on the resonant frequency of the mirror andthe resistance received from the ambient gas in which the mirrorvibrates.

[0070] To suppress this transient response, a triangular bias pulse maybe added onto the bias voltage at about the time the mirror lands. Notethat mirror land times may vary in a range of about, e.g., 1-2 μsec dueto various factors such as hinge memory torqueing. Accordingly, atriangular pulse may be used because its effects are generallyindependent of the variety of landing times and of the phase of theresonant vibration. Referring now to FIG. 12a, there is shown atriangular pulse time domain function 120 and its associated spectrumdensity function 122. For comparison, FIG. 12b shows a rectangular pulsetime domain function 124 and its associated spectrum density function126. Triangular pulse function 120 generally can provide a wider rangefor filtering at, e.g., null frequency 128 compared with rectangularpulse function 124, which exhibits a steep rate of change at the nullfrequency 129.

[0071] By setting the null frequency of the triangular pulse function tothe resonant frequency of the mirrors, the resonant vibration of themirrors generally is not affected by electrostatic function forcegenerated by the triangular pulse function on the bias voltage curve.Therefore, a triangular pulse function generally would only act on themirrors to suppress their vibration even though the mirrors land atdifferent times.

[0072] With reference now to FIG. 13a, there is shown a damped resetsequence 130 comprising a damping triangular pulse 132 for resonantvibration suppression of a DMD mirror such as the mirror 140 shown inFIG. 14. Damping triangular pulse 132 generally will damp the mirrortransient response independent of the variation in the phasing of themirror transient resonant vibration. The triangular shape of the pulsein the time domain is designed to have a zero frequency component nearthe transient resonant frequency of the mirror. Because the triangularpulse has other frequency components ranging from DC to higherfrequencies except for the frequency near the transient resonantvibration, the pulse will dump the mirror vibration by the increase ofelectrostatic attractive force between mirror and address electrodes.

[0073] Assuming a resonant frequency of 550 kHz for the mirrors, tau oftriangular pulse 132 may be set to, e.g., 1.82 μsec. This value may bemodified depending on the specific application and its actual resonantfrequency. For example, tau may be set to about 2.22 μsec for a 450 kHzresonant frequency. In addition, the number of triangular pulses may bevaried from the singular pulse shown in FIG. 13a. For example, there maybe two pulses as shown in the damped reset sequence 134 of FIG. 13b. Thenumber of pulses, the pulse width, and the pulse energy may varydepending on the specific application and may be chosen to maximizetheir effect on transient resonant vibration suppression and onstabilization.

[0074] The effect of a single triangular pulse is illustrated in FIG.15. In this graph, a damped reset sequence 150 is shown with a dampingtriangular pulse 152. The mirror angle response to this sequence is alsoshown, both for crossover trajectory angle 154 and stay trajectory 156.In the figure, the solid line crossover trajectory 158 and the dashedline stay trajectory 160 represent the mirror angle response to avoltage function with no damping pulse. Likewise, dotted line 162represents the crossover trajectory response and the dotted line 164represents the stay trajectory response to a voltage function with thedamping pulse 152. As can be seen in FIG. 15, the damping pulsesuppresses mirror resonance after landing, stabilizing the mirror morequickly and enabling a shorter mechanical switching time.

[0075] Although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, many of the features and functions discussed above can beimplemented in software, hardware, or firmware, or a combinationthereof. As another example, it will be readily understood by thoseskilled in the art that pulse widths, voltage levels and voltagepolarities may be varied while remaining within the scope of the presentinvention. In addition, the damping pulses disclosed herein may be usedwhen driving a mirror to a neutral position or to a landing position,and the features described with respect to one embodiment may beutilized by other disclosed embodiments.

[0076] Moreover, the scope of the present application is not intended tobe limited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A method of operating a digital micromirrordevice having at least one micromirror, the method comprising: applyinga reset voltage pulse to the micromirror, wherein the reset voltagepulse causes the micromirror to launch from a landing plate; applying anoffset voltage to the micromirror immediately after the reset voltagepulse, wherein the offset voltage is applied for a damping delay period,wherein the micromirror launches and is moving away from the landingplate before an expiration of the damping delay period; applying adamping pulse to the micromirror immediately after the offset voltage;and reapplying the offset voltage to the micromirror, whereby thedamping pulse reduces oscillation of the micromirror about a neutralposition.
 2. The method of claim 1, wherein the damping pulse has thesame polarity as the reset voltage pulse.
 3. The method of claim 1,wherein the damping pulse has the same polarity and magnitude as thereset voltage pulse.
 4. The method of claim 1, further comprising:loading an address state for the micromirror during the reapplying ofthe offset voltage; and applying a bias voltage to the micromirror,wherein the micromirror assumes the address state.
 5. The method ofclaim 1, wherein the reset voltage is about −26 volts and the offsetvoltage is about 7 volts.
 6. The method of claim 5, wherein the dampingpulse voltage is about −26 volts.
 7. The method of claim 1, wherein thedamping delay period is greater than 1 microsecond.
 8. The method ofclaim 7, wherein the damping delay period is about 1.6 microseconds andthe damping pulse is about 3.9 microseconds long.
 9. A method ofoperating a digital micromirror device having at least one micromirror,the method comprising: applying a reset voltage pulse to themicromirror, wherein the reset voltage pulse causes the micromirror tolaunch from a landing plate; applying an offset voltage to themicromirror immediately after the reset voltage pulse, wherein theoffset voltage is applied for a damping delay period, wherein themicromirror launches and is moving away from the landing plate before anexpiration of the damping delay period; applying a damping pulse to themicromirror immediately after the offset voltage, wherein the dampingpulse voltage is the same as the reset voltage; and reapplying theoffset voltage to the micromirror, whereby the damping pulse reducesoscillation of the micromirror about a neutral position.
 10. The methodof claim 9, further comprising: loading an address state for themicromirror during the reapplying of the offset voltage; and applying abias voltage to the micromirror, wherein the micromirror assumes theaddress state.
 11. The method of claim 9, wherein the reset voltage isabout −26 volts and the offset voltage is about 7 volts.
 12. The methodof claim 9, wherein the damping delay period is greater than 1microsecond.
 13. The method of claim 12, wherein the damping delayperiod is about 1.6 microseconds and the damping pulse is about 3.9microseconds long.